1. Field of the Invention
This invention generally relates to a method and a system for reforming soft soil such as muddy soil or swampy soil by draining underground water therefrom. More particularly, this invention relates to a method and a system for efficiently hardening soft soil by adequately separating air passages and water passages in the drainage routes.
2. Background Art
JP Patent Application Laid-Open No. 11-131465 teaches use of vertical drains which are laid vertically in target soft soil to vacuum the soil and drain underground water.
FIG. 11 shows a conventional drain system comprised of vertical drains 1, horizontal drains 2 laid in contact with the vertical drains 1, water gathering pipes 3 laid in contact with the horizontal drains 2, and airtight sheet means 6 which covers target soft soil A after the vertical drains 1, the horizontal drains 2 and the water gathering pipes 3 have been installed in place. The drain system is further comprised of vacuum tank means 4 placed in connection with the water gathering pipes 3 and vacuum pump means 5 placed in connection with the vacuum tank means 4.
In use of the conventional drain system shown in FIG. 11, the vacuum pump means 5 vacuums the vacuum tank means 4. When a check valve (not shown) provided on the vacuum tank means 4 is opened, the water gathering pipes 3 are vacuumed. The vacuuming effect propagates to the horizontal drains 2 and the vertical drains 1 which are in connection with the horizontal drains 2 and reduces their respective inner pressures to below 0.4 atm. The target soft soil A is gradually evacuated from around the vertical drains 1 where air is drawn into the vertical drains 1. The evacuated regions gradually spread throughout the soft soil A.
Spread of the evacuated regions in the soft soil A directs underground water and underground air towards the vertical drains 1 and the water and air drawn into the vertical drains 1 travel up through the vertical drains 1. The water and air are sucked into the horizontal drains 2 and then into the water gathering pipes 3. The continued drainage of the target soft soil A further spreads the evacuated regions.
The whole of the target soft soil A will eventually be vacuumed to around 0.4 atm, and underground water and air are eventually drained out of the soft soil A, leading to compaction of the soft soil A to a harder and stabler soil state.
It is to be noted that in the conventional system the vacuuming routes and water drain routes are common. Therefore, sucked air and water flow together all through the common routes comprised of the vertical drains 1, the horizontal drains 2, the water gathering pipes 3, the vacuum tank means 4 and the vacuum pump means 5.
Initially, underground water and air flow into the water gathering pipes 3 in large quantities from the horizontal drains 2, which stuffs the water gathering pipes 3 and therefore impedes subsequent vacuuming of the target soft soil A as will be readily understood by those skilled in the art.
In addition, as the compaction of the soft soil A progresses and the soft soil A sinks, the vertical distance from the underground water level to the vacuum pump means 5 widens, and efficiency of drainage degrades as will be readily understood by those with ordinary skills.